After satisfying eligibility criteria, subjects will be randomly assigned to initial active treatment or placebo following a 4 week run-in period. After completion of period 1 and a 4 week washout period, subjects will cross-over to the alternative treatment. After 4 weeks of period 2, subjects will undergo final assessment as shown in the diagram below. However participants choosing not to continue taking ivacaftor in a Named Patient Program or another similar program run by Vertex Pharmaceuticals, Inc., ('Vertex'), at the end of the study period will be required to undergo a Safety Follow-Up visit 28 days after the final dose of study drug. Safety Follow-Up assessments will not include the CPET.